The present invention relates to a scanning optical system having an optical system for detecting an information beam.
For the scanning optical system in which the scanning beam is deflected by a rotary polygonal mirror, oscillating mirror or the like to scan the scanning plane, it has been known in the art to take up a part of the scanning beam as an information beam and to detect various information from the beam.
For example, such information beam is used for timing the commencement of scanning. When the beam deflecting device is a polygonal mirror that is not so good in accuracy of dividing the polygonal surfaces, there arises difficulties regarding timing the commencement of scanning i.e. synchronizing the time at which every scanning line should commence indicating or recording image on the scanning plane. One of the difficulties is known as so-called "jitter". When signal is carried by the scanning beam deflected by such a polygonal mirror having inaccurately finished polygonal surfaces, the position at which every scanning line carries out writing or reading of signal is displaced relative to the direction of scanning variously with every scanning line according as the angular error in the associated surface of the polygon. This distortional phenomenon is called "jitter". This problem of "jitter" becomes important when high accuracy in indicating or writing an image is required. It is possible to prevent the "jitter" to some extent by eliminating the angular errors in the polygonal surfaces. However, the manufacture of polygonal mirrors having high accuracy is very difficult. In particular this is true for the manufacture of polygonal mirrors having a great number of polygonal surfaces. For this reason, instead of increasing the accuracy in finishing the polygonal surfaces, an information beam is taken up from the scanning beam and used for timing the scanning as described above. Also, such information beam is useful to detect whether the scanning beam from the light source is properly oscillating or whether the beam deflecting device such as a rotary polygone mirror is operating correctly at a predetermined speed. Furthermore, by measuring the light value of the information beam, the luminance of the light source can be detected. The information obtained from such detection of the information beam is fed back to continuously adjust the operation of scanning to its normal state.
A typical example of the optical system hitherto known and used for taking up such an information beam is illustrated in FIG. 1. The optical system shown in FIG. 1 is an information beam taking-up optical system by which an information beam for synchronizing signal as mentioned above is taken up. The information beam is taken up from the light deflected by a beam deflecting device and passed through a scanning focusing lens, but from a portion of the light other than the effective beam portion of the same. Herein the term "effective beam" is defined as the light beam that is used to scan that area of the scanning plane the scanning of which is absolutely necessary (hereinafter said area is referred as "essential scanning area").
Now referring to FIG. 1, reference numeral 1 designates a laser beam source. The beam emitted from the beam source is introduced into a beam modulator 3 which modulates the beam in accordance with the signal coming from a control circuit 2. The modulated beam is directed to a beam expander 4 where the diameter of the beam is enlarged. Now the expanded beam is inpinged upon a rotary polygonal mirror 5. The polygonal mirror 5 deflects the beam to a focusing lens 6 which focuses the beam on a scanning plane 7 that is an indicating (displaying) surface or a recording surface. Reference numeral 8 designates a light receiving element (or photo element). The light receiving element is arranged in such manner that at every scanning, the scanning image from the rotary polygonal mirror 5 has been received by the element 8 before the image reaches the recording surface 7.
To obtain the necessary synchronizing signal, a part of the reflected beam from the polygonal mirror 5 is received by the light receiving element 8 as an information beam 9 after it is passed through the focusing lens 6 and a knife edge 10.
When the beam is received by the light receiving element 8, its excitation is detected by a detector 11 so as to actuate a timer circuit. After a predetermined time duration, the control circuit 2 starts operating so that signals in the amount corresponding to that of one scanning are successively sent into the beam modulator 3. This timing operation is repeated for every surface of the polygonal mirror 5. This enables correction of the adverse effect on scanning caused by the lack of uniformity of divided surfaces on the polygon and, therefore, the desired image indication or recording without "jitter" and having well aligned fronts is attainable.
However the information beam take-up system as illustrated in FIG. 1 and described above has some important drawbacks.
Among others, it necessitates a large angle of field for the focusing lens 6. This is because the information beam must be passed through the focusing lens and taken up from a portion of the light beam other than the effective beam. To form a large angle of field, a focusing lens having a large diameter and also a large sized polygonal mirror 5 should be used. This is disadvantageous in view of the cost and difficulty in manufacture, and also forms an obstacle to making the apparatus more compact.
There is a case where the focusing lens 6 should be a lens having a small angle of field and a short focal length. For such a case, the conventional system described above is very inconvenient. For example, this may apply to the case where writing or indication should be carried out on a scanning plane having a very small scanning width such as a micro-film with a scanning beam having high resolving power. For the radius .psi. of the spot diameter of a scanning beam pencil on the scanning plane, the following equation is given: EQU .psi. = 1.22 .times. (.lambda./sin .beta.)
wherein,
.lambda. is the wave length of the scanning beam and PA1 .beta. is the convergent angle of the scanning beam focused the scanning plane. PA1 Type 1. The direction of the exit beam from the member is the same as that of the incident beam to it. PA1 Type 2. The direction of the exit beam from the member is opposite to that of the incident beam to it. PA1 Type 3. The exit beam is directed to the desired direction in the plane containing the incident beam or a plane parallel with said plane relative to the direction of the incident beam.
Provided that .lambda. is constant, the use of a focusing lens having a shorter focal length will result in a larger value of sin .beta. and, therefore, a smaller value of .psi.. In other words, by using a focusing lens having a shorter focal length, a scanning beam having a higher resolving power and a smaller spot diameter is obtained. Also, it is known that a focusing lens having a focal length of f focuses a beam with a field angle of .theta. on the scanning plane at the position, y = f..theta. wherein y is the height of image measured from the optical axis of the focusing lens taken as origin. Now, provided that the number of resolution points required per scanning line is N and the maximum angle of field of the focusing lens is .theta..sub.max (one side), angle per point i.e. resolution angle will be represented by 2.multidot.f.multidot..theta..sub.max /N. Accordingly, the following equation is given: EQU l = 2.multidot.f.multidot..theta..sub.max /N
wherein l is the size on the image surface per point. Therefore it will be understood that the smaller the focal length f or the maximum angle of field .theta..sub.max is, the less value of l is given. This means that because of the smaller value of the image size per point (l), a more strict accuracy in positioning is required for mounting the light receiving element. For example, vibrational displacement of the element including the knife edge in the scanning direction even in the order of 1.mu..about.0.1.mu. is not permissible for the scanning of micro-film described above.
Furthermore, when the focal length of focusing lens is small, the focusing intensity of the focal point becomes necessarily small. For this reason, a strict accuracy in positioning in the direction of scanning beam propagation is also required for mounting the knife edge. Usually the length of the light receiving element at its light receiving portion is in the order of 0.5 mm or more, which is extremely larger than the diameter of the scanning beam. Therefore, there may be caused synchronization error by any change in output due to the ununiformity of the light receiving part.